简单工厂模式----控制台计算器的设计(C++实现)

        一个计算器的设计却也体现出了面向对象设计的封装、继承与多态三大特性。

       让计算数据和过程与显示结果分开体现了封装的特性；为了程序的可扩展性，比如以后要加入开根号的运算而不去重写整个类，可以将每个操作符单独作为一个类，他们都继承共同的父类------抽象类：Operation类，这里体现了继承和多态的特性。

       如何让计算器知道该用哪个操作符的类呢，这里用到了“简单工厂模式”，也就是，到底要实例化哪个操作符类，应该考虑用一个单独的类来做这个创造实例的过程，这就是工厂。

#include <iostream>
#include <exception>

using namespace std;

class Operation
{
public:
virtual double GetResult() = 0;

void SetNumberA( double a )
{
numberA = a;
}
double GetNumberA()
{
return numberA;
}

void SetNumberB( double b )
{
numberB = b;
}
double GetNumberB()
{
return numberB;
}

protected:
double numberA;
double numberB;
};

class OperationAdd : public Operation
{
public:
virtual double GetResult()
{
return numberA + numberB;
}
};

class OperationSub : public Operation
{
public:
virtual double GetResult()
{
return numberA - numberB;
}
};

class OperationMul : public Operation
{
public:
virtual double GetResult()
{
return numberA * numberB;
}
};

class OperationDiv : public Operation
{
public:
virtual double GetResult()
{
if( -0.00000001 < numberB && numberB < 0.00000001 )
throw exception("除数不能为0");

return numberA / numberB;
}
};

class OperationFactory
{
public:
OperationFactory()
{
operation = NULL;
}

Operation* CreateOperate( char o )
{
switch( o )
{
case '+':
operation = new OperationAdd();
break;
case '-':
operation = new OperationSub();
break;
case '*':
operation = new OperationMul();
break;
case '/':
operation = new OperationDiv();
break;
default:
throw exception("操作符错误");
}

return operation;
}

~OperationFactory()
{
delete operation;
}

private:
Operation* operation;
};

int main()
{
double numberA;
double numberB;
char operate;

cout<<"输入数字A: ";
cin>>numberA;

cout<<"运算符(+ - * /): ";
cin>>operate;

cout<<"输入数字B: ";
cin>>numberB;

OperationFactory factory;
Operation* operation;

try
{
operation = factory.CreateOperate( operate );
}
catch(exception& e)
{
cout<<e.what()<<endl;
exit(1);
}
operation->SetNumberA( numberA );
operation->SetNumberB( numberB );

int result;
try
{
result = operation->GetResult();
}
catch( exception& e)
{
cout<<e.what()<<endl;
exit(1);
}
cout<<"计算结果："<<result<<endl;

delete operation;

return 0;
}